The process for fabricating optical lenses, and more particularly corrective ophthalmological lenses, requires particularly high levels of care and precision. It generally comprises two main steps. Initially a semi-finished lens, also referred to as an optical blank or preform, is obtained by molding the synthetic or inorganic material that has been selected to constitute the basic substrate of the lens. Thereafter, the molded semi-finished lens is surfaced on one and/or both of its optically-useful main faces in order to satisfy the geometrical model and the prescribed correction. Because of the exacting requirements in terms of precision and roughness to which lenses are subjected, this surfacing operation is subdivided into a plurality of substeps associated with a corresponding number of specific workstations, so it becomes necessary to organize the transfer of the lens being surfaced from one station to another. Thus, for surfacing each of the faces of the lens, there are generally to be found a machining station that serves both to blank out and to finish by using two distinct tools, and a polishing station, possibly preceded by a smoothing station. Between these stations, and after them, there may be other stations for working on or inspecting the lens, for example an etching station, a station for inspecting shape or appearance, etc.
One of the more specific problems encountered during this process of surfacing the lens lies in assembling the lens on each station in a position that is precise and well-controlled. This repeated intermediate operation of taking hold of the part again and again, commonly known as “blocking” the lens, is particularly difficult and expensive and often leads to imprecise positioning of a kind that can significantly degrade the optical quality of the finished lens. Such blocking of the lens suffers from two constraints that are cumulative and antagonistic.
Firstly, the lens, which is constituted of transparent synthetic or inorganic material that has not yet been varnished, is relatively fragile and must be protected from any marking or cracking, particularly on that one of its two faces that has been finished while work is taking place on its other face. The risk of marking is particularly pronounced with synthetic materials.
In addition, and above all, the lens must be positioned on each station concerned in a manner that is very precise, so that it has a known and stable three-dimensional orientation in a determined frame of reference of the station in question. This constraint concerning geometrical stability of the blocking is particularly awkward and difficult to comply with when fabricating lenses having surfaces that are complex, such as progressive or personalized lenses that do not present circular symmetry. It will be understood that the surfacing of such lenses is accompanied by variations in cutting forces on gradients that are steep, and as a result it leads to deformation, accompanied by relative geometrical instability of the blocking of the lens.
Several ways are known for “blocking” a semi-finished lens or blank in order to mount it and rotate it on machine tools or measuring devices of different workstations, and in particular of surfacing stations. Traditionally, a blocking support is used, sometimes also referred to as a grip block or chuck, possessing firstly blocking means for receiving and holding the lens via one of its main faces, and secondly means for securing the support on the nose of various machine tools or measurement and inspection devices so as to provide blocking of the lens on the machine or device, possibly accompanied by the lens being driven in rotation.
The main difficulty lies in the way in which the lens is blocked on the support, given the above-mentioned constraints.
The method that is in most widespread use at present, because of its geometrical precision, consists in forming and securing a metal block on one of the faces of the lens by casting thereon a molten alloy having a low melting temperature, the metal block constituting the blocking support and presenting means enabling it to be secured to the noses of the machine tools in the various workstations involved. That method generally gives satisfaction in terms of precision and stability, but it presents several drawbacks economically and environmentally that make it necessary to seek alternative blocking means. The low melting point alloys used are relatively expensive and should be considered as pollutants that are dangerous for the environment, such that it is necessary both for economic reasons and for ever-increasing environmental constraints, to organize meticulous recycling thereof. However even with efficient recycling, it is not possible to avoid loosing alloy by evaporation during melting. Furthermore, because of the relative complexity of the operation and because of its cost, in particular given the above-mentioned environmental aspects, it is common practice to keep the lens blocked on the same support throughout the process, the assembly constituted by the lens and its support being transferred from station to station. Unfortunately, the assembly is relatively bulky, such that handling it, transporting it, and storing it all lead to additional logistics costs. Furthermore, for technical reasons, there also exists a minimum length of time that must elapse before a lens associated with its holding block can be fitted to a machining station (about 15 minutes), and a maximum length of time beyond which machining can no longer be performed (about 24 hours); these times thus put constraints on the work flows of said lenses. In addition, in the event of prolonged storage or waiting between two operations, it is excessively expensive to accommodate holding blocks in progress in quantities equivalent to the quantities of lenses in progress.
That is why it is sometimes necessary between two operations to release a lens from its initial support in order to transfer it, store it, or transport it more easily. When the process restarts, it is necessary to associate the lens with a new holding block, with the practical difficulties that stem therefrom not only in terms of casting the low-melting point alloy and recycling it, but also in terms of achieving complete geometrical control over such a restarted part, and the associated extra costs.
In order to avoid using a molten metal alloy, proposals have been made to use a wax, for example, to bond a lens to a corresponding face of the blocking support that has approximately the same curvature. However that solution, like using a block of fusible metal, leads to practical difficulties relating to release, i.e. separating the lens from the support, and to cleaning the lens, with the environmental repercussions that stem therefrom. Above all, the precision and the stability of the bonding between the lens and the support can turn out to be insufficient. The shape of the layer of adhesive or wax interposed between the lens and the support includes a random contribution, or is in any event difficult to control and can suffer from deformation in compression and in twisting during surfacing operations under the effect of stresses generated by the surfacing tool.
Finally, lens blocking systems have been proposed that rely on pneumatic suction. Such systems make use of a pneumatic chuck or grip block that, in order to form a kind of control suction cup, presents a cavity surrounded by an annular gasket against which the preform is pressed in order to co-operate with the cavity and the gasket to define a chamber in which suction is established. The suction may be created either in a vacuum vessel that for the blocking operation contains both the lens and the grip block, or else by means of a vacuum pump connected to the cavity in the block via a pneumatic valve.
That solution of pneumatic blocking, also referred as vacuum blocking, does not present the same economic and environmental drawbacks as the above-described solutions involving blocks that are cast or bonded by adhesive. Implementing the vacuum solution is particularly quick and simple both during blocking and during release, and no chemical consumable is involved. Nevertheless, in spite of its considerable advantages, that type of blocking is little used in practice. It is found to be lacking in the precision and the stability with which the lens is secured, to an extent that is analogous to that which is observed when using supports with adhesive. The solution is found to be particularly difficult to implement with surfaces that are complex (i.e. not spherical or toroidal) against which the elastically-compressible gasket does not press in a manner that is sufficiently precise and stable. It would indeed be possible to increase the stiffness of the compressibility of the gasket, but that would be to the detriment of its coefficient of friction and would lead to a reduction in the torque that can be transmitted when rotating the lens, unless the pressure in the suction chamber is reduced so as to increase the magnitude of the suction effect exerted by the support on the lens, but that would run the risk of deforming the lens. It has also been found that insufficient torque transmission runs the risk of slip, in particular while a lens being processed is in rotation. Such slip is liable to spoil the final positioning of the lens in front of the user's eyes, which is particularly harmful in terms of the user's visual comfort, particularly with progressive ophthalmological lenses.
U.S. Pat. No. 3,794,314 describes a pneumatic blocking support for blocking an optical lens on a machine or device, the support possessing firstly blocking means for receiving and holding one face of the optical lens, and secondly means enabling it to be secured to a corresponding member of the machine or device, said blocking means comprising a central cavity and a gasket having at least one annular portion against which the lens is pressed in order to co-operate with said cavity and said gasket to define a suction chamber, the blocking means including abutment means arranged to provide the optical lens with a seat that is rigid once the gasket has deformed elastically. However, in that support, the gasket is arranged to deform in bending and therefore acts like a lip seal.
That arrangement does not resolve all of the above-mentioned drawbacks. It suffers from three major drawbacks. Firstly it limits the contact area between the gasket and the lens, which presses solely against the free edge (inner edge) of the gasket. That narrow contact area tends to reduce the maximum torque that can be transmitted, so the risk of slip remains. Secondly, it does not make it easier to find a satisfactory compromise between stiffness and coefficient of friction, since its work in bending tends to impose high bending stiffness, whereas the desire for a high coefficient of friction tends on the contrary to look for an elastomer with limited stiffness. Torque transmission is therefore difficult to increase by selecting an appropriate material. Thirdly, working in bending leads to the elastomer wearing quickly, particularly when the elastomer possesses high stiffness.